.img-box {
    width: 100%;
    display: flex;
    flex-flow: wrap;
}
.img-box-item {
    flex: 0 0 33.333%;
    height: 200px;
    padding: 5px;
    box-sizing: border-box;
    img {
        display: block;
        width: 100%;
        height: 100%;
    }
}